What is the difference between porpoises and dolphins?

Porpoises are generally smaller than dolphins. With the exception of the finless porpoise, they have a triangular dorsal fin (whereas most dolphins have a sickle-shaped fin). Porpoises also have different teeth; they are spade-shaped and look much more like our teeth, whereas dolphins have cone-shaped teeth (which looks like an upside-down ice-cream cone). Dolphins can make some sounds for communication within our hearing range (which ends at about about 20 kHz), whereas porpoises communicate in the ultrasonic range.